Add documentation for task management system

This syncs documentation from cloudflare/agents PR #683 which adds a task management system with lifecycle tracking to the Agents SDK.

Changes include:
- New concept page explaining tasks and their lifecycle
- API reference documentation for the Tasks API
- Tutorial guide for building a task-based agent
- Examples of using @task() decorator and TaskContext
- Client-side task tracking with useTask() hook

Related PR: cloudflare/agents#683

Author: github-actions[bot]